1. Field of the Invention
This invention relates generally to automotive ignition systems, and more specifically to a multistrike ignition system which produces a train of ignition sparks at the spark gaps of an internal combustion engine in proper timed sequence during a demanded firing duration, which is defined as the lapsed time during which multisparks will occur at the spark plug.
2. Description of the Prior Art
A conventional multi-strike ignition system is disclosed in the U.S. Pat. No. 3,489,129, wherein a charge circuit for charging a ignition capacitor from a DC voltage converter includes a resistor and a first thyristor used as a charge switch. A discharge circuit, for discharging the ignition capacitor to a primary winding of a ignition transformer, has a second thyristor used as a iischarge switch. After the first thyristor is turned on, a charge current flows to the ignition capacitor from the DC voltage converter via the resistor and the first thyristor for a period of time determined by the time constant of the combination of the ignition capacitor and the resistor. As the result, the first thyristor operated as the charge switch cannot be turned off until the current flowing through the first thyristor is reduced due to the characteristic of the thyristor. The conditions necessary to turn off the thyristor include whether the current flowing through the thyristor is very small or whether a reverse voltage is applied to the thyristor. Therefore, the interval for charging the ignition capacitor is much longer and hence a timing for discharging the lgnition capacitor to the primary winding of the ignition transformer is delayed whereby the duty cycle of the multi-strike spark is reduced.
Furthermore, in the conventional ignition system, a timed sequence control for operating the charge circuit of the ignition capacitor and the discharge circuit of the ignition capacitor is operated by an oscillator which oscillates with a predetermined frequency without regard to the states of the thyristors.
Therefore, if the first thyristor is operated as a charge switch it may to be turned on by the oscillator in spite of the turned on state of the second thyristor which is being operated as the discharge switch. This is due to the turned on period of the second thyristor when misfiring occurred at the spark plug. The primary winding of the ignition transformer is directly loaded with the DC voltage from the DC voltage converter, whereby the DC voltage converter is fully discharged. Subsequently, the recovery time of the DC voltage converter, to return to the predetermined firing voltage is much longer and, therefore, the spark plug cannot be fired during this recovery time.
In the conventional multi-strike ignition system, the firing duration is fixed to a predetermined value and is independent of the rotational speed of the engine. In order to stabilize engine combustion and reduce the consumption of electrical energy, a firing duration control, in response to the rotational speed, is required. For example, the firing duration may be increased in response to the decrease in the rotational speed of the engine because the compressed fuel air mixture within the combustion chamber of the engine is less combustible at low speed conditions due to a low mixture swirl speed or low temperature of the combustion chamber. Conversely the firing duration may be made to decrease in response to the increasing of the rotational speed of the engine because the compressed fuel-air mixture within the combustion chamber of engine is more combustible at high speed condition of engine due to the high temperature in the combustion chamber.
Additionally, in the conventional multi-strike ignition system, and especially the multi-strike ignition system using the ignition capacitor, the ignition transformer is a high leakage inductance type transformer having a air gap. The type of transformer which has an air gap is generally used in an inductive discharge ignition system which stores the spark energy in the form of magnetic energy in the air gap. This particular type of transformer is often used as a part fo a capacitive discharge ignition system having the above described ignition capacitor. The transformer is used because of the economic considerations.
The air gap is necessary in order to provide storage of energy in the inductive discharge ignition system. On the other hand in capacitive discharge ignition systems which utilize the ignition capacitor, the air gap is not necessary in order to store energy because the transformer operates in that particular mode, as an energy transmitter instead of an energy storage device. Although leakage inductance of the ignition transformer due to the proper air gap is necessary for capacitive discharge ignition systems, because primary current flows through the primary winding of the ignition transformer is produced by the resonance of the leakage inductance and the ignition capacitance. Thus, spark current (reflection of primary current) may be of a correct value due to the correct leakage inductance value. However, it is to be noted that this kind of leakage inductance dependency has disadvantages with respect to the size of the transformer, because the voltage across the primary winding is too high even for the sustaining period. This means a large size core cross-section is required.
If the air gap which is utilized for leakage inductance of the ignition transformer in the capacitive discharge system is deleted, another problem occurs with respect to the low leakage inductance. Since the primary current is too high and the spark duration for one pulse is too short.